Low carbon steel sheet, low carbon steel cast piece and method for production thereof

ABSTRACT

The present invention provides a low carbon steel sheet and a low carbon steel slab on which formation of surface defects can be surely prevented by preventing aggregation of inclusions in the molten steel and finely dispersing inclusions in the steel sheet or slab, and a process for producing the steel sheet and slab. The present invention provides a process comprising the steps of decarburizing a molten steel so as to produce a carbon concentration of up to 0.01% by mass, pre-deoxidizing the molten steel by adding Al thereto so as to produce a dissolved oxygen concentration from 0.01 to 0.04% by mass, adding thereto Ti and at least La and/or Ce, and casting the molten steel, and a steel sheet and a steel slab obtained by the process.

TECHNICAL FIELD

[0001] The present invention relates to a low carbon steel sheet and alow carbon steel slab which are excellent in workability and formabilityand on which surface defects are hardly formed, and a process forproducing the same.

[0002] In addition, the term “low carbon” in the present inventionparticularly defines no upper limit of a carbon concentration, butsignifies that the carbon concentration is relatively low in comparisonwith other steel types. In addition, because a steel sheet is used forapplications in which the steel sheet is particularly severely worked,for example, external plates of automobiles, the steel sheet must bemade to have workability. The carbon concentration is therefore up to0.05% by mass, preferably up to 0.01% by mass. The lower limit of acarbon concentration is not particularly defined.

BACKGROUND ART

[0003] A molten steel having been refined with a converter or a vacuumtreatment vessel contains a large amount of dissolved oxygen. Theexcessive oxygen is generally deoxidized with Al that is a strongdeoxidizing element having strong affinity for oxygen. However, Al formsAl₂O₃ inclusions through deoxidation, and the inclusions aggregate toform coarse alumina clusters each having a size of at least severalhundreds of micrometers. The alumina clusters cause generation ofsurface defects during the production of a steel sheet, and greatlydeteriorate the quality of the steel sheet. In particular, for a lowcarbon molten steel as a material for a steel sheet that has a lowcarbon concentration and, after refining, a high dissolved oxygenconcentration, the amount of alumina clusters is very large. Thegeneration ratio of surface defects of a steel sheet produced therefromis extremely high, and measures for decreasing the Al₂O₃ inclusionsremain as an important problem to be solved.

[0004] In order to decrease the Al₂O₃ inclusions, the following methodshave heretofore been proposed and carried out: a method described inJapanese Unexamined Patent Publication (Kokai) No. 5-104219 comprisingadding flux for adsorbing inclusions on a molten steel surface to removeAl₂O₃ inclusions; and a method described in Japanese Unexamined PatentPublication (Kokai) No. 63-149057 comprising adding CaO flux to a moltensteel with the flow of pouring the molten steel utilized whereby Al₂O₃inclusions are adsorbed and removed. On the other hand, JapaneseUnexamined Patent Publication (Kokai) No. 5-302112 discloses, not as amethod of removing Al₂O₃ inclusions but as a method of not formingAl₂O₃, a method of preparing a molten steel for a steel sheet, whichcomprises deoxidizing a molten steel with Mg and in which the moltensteel is substantially not deoxidized with Al.

[0005] However, it is very difficult to remove Al₂O₃ inclusions formedin a low carbon molten steel in a large amount and to such a degree thatsurface defects are not formed, by the methods of removing Al₂O₃inclusions explained above. Moreover, when a molten steel is deoxidizedby Mg deoxidation that forms no Al₂O₃ inclusions, the Mg vapor pressureis high, and the yield of Mg in the molten steel is very low.Accordingly, a large amount of Mg is required to deoxidize, with Mgalone, a molten steel having a high dissolved oxygen concentration suchas a low carbon steel. Therefore, it cannot be said that the process ispractical when the production cost is taken into consideration.

[0006] In view of these problems, an object of the present invention isto provide a low carbon steel sheet and a low carbon steel slab on whichformation of surface defects can be surely prevented by preventingaggregation of inclusions in the molten steel and finely dispersinginclusions in the steel sheet or slab, and a process for producing thesteel sheet and slab.

DISCLOSURE OF THE INVENTION

[0007] The present invention has been achieved to solve the aboveproblems, and the aspects of the invention will be explained below.

[0008] (1) A low carbon steel sheet characterized in that fine oxideinclusions having a diameter from 0.5 to 3.0 μm are dispersed thereinwith the number being from not less than 1,000 to less than 100,000pieces/cm².

[0009] (2) A low carbon steel sheet characterized in that not less than60% by mass of oxide inclusions present therein contain at least Laand/or Ce.

[0010] (3) A low carbon steel sheet characterized in that not less than60% by mass of oxide inclusions present therein are spherical orspindle-like oxide inclusions containing at least La and/or Ce.

[0011] (4) A low carbon steel sheet characterized in that not less than60% by mass of oxide inclusions present therein are oxide inclusionscontaining not less than 20% by mass of at least La and/or Ce in theform of La₂O₃ and/or Ce₂O₃.

[0012] (5) A low carbon steel sheet characterized in that not less than60% by mass of oxide inclusions present therein are spherical orspindle-like oxide inclusions containing not less than 20% by mass of atleast La and/or Ce in the form of La₂O₃ and/or Ce₂O₃.

[0013] (6) A low carbon steel sheet characterized in that fine oxideinclusions having a diameter from 0.5 to 30 μm are dispersed thereinwith the number being from not less than 1,000 to less than 100,000pieces/cm², and that not less than 60% by mass of the oxide inclusionscontain at least La and/or Ce.

[0014] (7) A low carbon steel sheet characterized in that fine oxideinclusions having a diameter from 0.5 to 30 μm are dispersed thereinwith the number being from not less than 1,000 to less than 100,000pieces/cm², and that not less than 60% by mass of the oxide inclusionsare spherical or spindle-like oxide inclusions containing at least Laand/or Ce.

[0015] (8) A low carbon steel sheet characterized in that fine oxideinclusions having a diameter from 0.5 to 30 μm are dispersed thereinwith the number being from not less than 1,000 to less than 100,000pieces/cm², and that not less than 60% by mass of the oxide inclusionsare oxide inclusions containing not less than 20% by mass of at least Laand/or Ce in the form of La₂O₃ and/or Ce₂O₃.

[0016] (9) A low carbon steel sheet characterized in that fine oxideinclusions having a diameter from 0.5 to 30 μm are dispersed thereinwith the number being from not less than 1,000 to less than 100,000pieces/cm², and that not less than 60% by mass of the oxide inclusionsare spherical or spindle-like oxide inclusions containing not less than20% by mass of La and/or Ce in the form of La₂O₃ and/or Ce₂O₃.

[0017] (10) A low carbon steel slab characterized in that fine oxideinclusions having a diameter from 0.5 to 30 μm are dispersed in thesurface layer of the slab from the surface to the depth of 20 mm withthe number being from not less than 1,000 to less than 100,000pieces/cm².

[0018] (11) A low carbon steel slab characterized in that not less than60% by mass of oxide inclusions present in the surface layer of the slabfrom the surface to the depth of 20 mm contain at least La and/or Ce.

[0019] (12) A low carbon steel slab characterized in that not less than60% by mass of oxide inclusions present in the surface layer of the slabfrom the surface to the depth of 20 mm are spherical or spindle-likeoxide inclusions containing at least La and/or Ce.

[0020] (13) A low carbon steel slab characterized in that not less than60% by mass of oxide inclusions present in the surface layer of the slabfrom the surface to the depth of 20 mm are oxide inclusions containingnot less than 20% by mass of at least La and/or Ce in the form of La₂O₃and/or Ce₂O₃.

[0021] (14) A low carbon steel slab characterized in that not less than60% by mass of oxide inclusions present in the surface layer of the slabfrom the surface to the depth of 20 mm are spherical or spindle-likeoxide inclusions containing not less than 20% by mass of at least Laand/or Ce in the form of La₂O₃ and/or Ce₂O₃.

[0022] (15) A low carbon steel slab characterized in that fine oxideinclusions having a diameter from 0.5 to 30 μm are dispersed in thesurface layer of the slab from the surface to the depth of 20 mm withthe number being from not less than 1,000 to less than 100,000pieces/cm², and that not less than 60% by mass of the oxide inclusionscontain at least La and/or Ce.

[0023] (16) A low carbon steel slab characterized in that fine oxideinclusions having a diameter from 0.5 to 30 μm are dispersed in thesurface layer of the slab from the surface to the depth of 20 mm withthe number being from not less than 1,000 to less than 100,000pieces/cm², and that not less than 60% by mass of the oxide inclusionsare spherical or spindle-like oxide inclusions containing at least Laand/or Ce.

[0024] (17) A low carbon steel slab characterized in that fine oxideinclusions having a diameter from 0.5 to 30 μm are dispersed in thesurface layer of the slab from the surface to the depth of 20 mm withthe number being from not less than 1,000 to less than 100,000pieces/cm², and that not less than 60% by mass of the oxide inclusionsare oxide inclusions containing not less than 20% by mass of at least Laand/or Ce in the form of La₂O₃ and/or Ce₂O₃.

[0025] (18) A low carbon steel slab characterized in that fine oxideinclusions having a diameter from 0.5 to 30 μm are dispersed in thesurface layer of the slab from the surface to the depth of 20 mm withthe number being from not less than 1,000 to less than 100,000pieces/cm², and that not less than 60% by mass of the oxide inclusionsare spherical or spindle-like oxide inclusions containing not less than20% by mass of at least La and/or Ce in the form of La₂O₃ and/or Ce₂O₃.

[0026] (19) A process for producing a low carbon steel slab comprisingthe steps of: decarburizing a molten steel so as to produce a carbonconcentration of up to 0.01% by mass; adding at least La and/or Cethereto so as to produce an adjusted dissolved oxygen concentration from0.001 to 0.02% by mass; and casting the molten steel.

[0027] (20) A process for producing a low carbon steel slab comprisingthe steps of: decarburizing a molten steel so as to produce a carbonconcentration of up to 0.01% by mass; adding thereto Ti and at least Laand/or Ce; and casting the molten steel.

[0028] (21) A process for producing a low carbon steel slab comprisingthe steps of: decarburizing a molten steel so as to produce a carbonconcentration of up to 0.01% by mass; pre-deoxidizing the molten steelby adding Al thereto so as to produce a dissolved oxygen concentrationfrom 0.01 to 0.04% by mass; adding thereto Ti and at least La and/or Ce;and casting the molten steel.

[0029] (22) A process for producing a low carbon steel slab comprisingthe steps of: decarburizing a molten steel so as to produce a carbonconcentration of up to 0.01% by mass; pre-deoxidizing the molten steelby adding Al thereto and stirring the molten steel for at least 3minutes so as to produce a dissolved oxygen concentration from 0.01 to0.04% by mass; adding thereto Ti in an amount from 0.003 to 0.4% by massand at least La and/or Ce in an amount from 0.001 to 0.03% by mass; andcasting the molten steel.

[0030] (23) A process for producing a low carbon steel slab comprisingthe steps of: decarburizing a molten steel with a vacuum degassingapparatus so as to produce a carbon concentration of up to 0.01% bymass; adding at least La and/or Ce thereto so as to produce an adjusteddissolved oxygen concentration from 0.001 to 0.02% by mass; and castingthe molten steel.

[0031] (24) A process for producing a low carbon steel slab comprisingthe steps of: decarburizing a molten steel with a vacuum degassingapparatus so as to produce a carbon concentration of up to 0.01% bymass; adding thereto Ti and at least La and/or Ce; and casting themolten steel.

[0032] (25) A process for producing a low carbon steel slab comprisingthe steps of: decarburizing a molten steel with a vacuum degassingapparatus so as to produce a carbon concentration of up to 0.01% bymass; pre-deoxidizing the molten steel by adding Al thereto so as toproduce to a dissolved oxygen concentration from 0.01 to 0.04% by mass;adding thereto Ti and at least La and/or Ce; and casting the moltensteel.

[0033] (26) A process for producing a low carbon steel slab comprisingthe steps of: decarburizing a molten steel with a vacuum degassingapparatus so as to produce a carbon concentration of up to 0.01% bymass; pre-deoxidizing the molten steel by adding Al thereto and stirringthe molten steel for at least 3 minutes so as to produce a dissolvedoxygen concentration from 0.01 to 0.04% by mass; adding thereto Ti in anamount from 0.003 to 0.4% by mass and at least La and/or Ce in an amountfrom 0.001 to 0.03% by mass; and casting the molten steel.

[0034] (27) The process for producing a low carbon steel slab accordingto any one of (19) to (26) wherein, during casting the molten steel, themolten steel is cast in a mold having an electromagnetic stirringfunction.

[0035] (28) The process for producing a low carbon steel slab accordingto any one of (19) to (26) wherein, during casting the molten steel, themolten steel is cast using mold flux having a viscosity of not lowerthan 4 poise at 1,300° C.

[0036] (29) The process for producing a low carbon steel slab accordingto any one of (19) to (26) wherein, during casting the molten steel, themolten steel is cast in a mold having an electromagnetic stirringfunction using mold flux having a viscosity of not lower than 4 poise at1,300° C.

[0037] (30) The process for producing a low carbon steel slab accordingto any one of (19) to (26) wherein, during casting the molten steel, themolten steel is continuously cast.

[0038] (31) The process for producing a low carbon steel slab accordingto any one of (19) to (26) wherein, during casting a molten steel, themolten steel is continuously cast in a mold having an electromagneticstirring function.

[0039] (32) The process for producing a low carbon steel slab accordingto any one of (19) to (26) wherein, during casting the molten steel, themolten steel is continuously cast using mold flux having a viscosity ofnot lower than 4 poise at 1,300° C.

[0040] (33) The process for producing a low carbon steel slab accordingto any one of (19) to (26) wherein, during casting the molten steel, themolten steel is continuously cast in a mold having an electromagneticstirring function using mold flux having a viscosity of not lower than 4poise at 1,300° C.

BEST MODE FOR CARRYING OUT THE INVENTION

[0041] The present invention will be explained in detail.

[0042] A molten steel decarburized with a converter and a vacuumtreatment vessel contains a large amount of dissolved oxygen. Becausemost of the dissolved oxygen is usually decreased by deoxidation withadded Al (reaction according to the formula (1)), a large amount ofAl₂O₃ inclusions is formed.

2Al+3O=Al₂O₃  (1)

[0043] These inclusions aggregate together immediately after deoxidationto form coarse alumina clusters each having a size of several hundredsof micrometers to cause surface defects during the production of a steelsheet.

[0044] For the purpose of forming no alumina clusters, the presentinventors have therefore paid attention to removing the dissolved oxygenby deoxidizing the molten steel subsequent to decarburization with adeoxidizing agent other than Al.

[0045] The present inventors have devised, as a process of theinvention, a process comprising the steps of: decarburizing a moltensteel so as to produce a carbon concentration of up to 0.01% by mass byrefining the molten steel with a steel making furnace such as aconverter or an electric furnace, and further subjecting the moltensteel to vacuum degassing procedure or the like; adding at least Laand/or Ce thereto so as to produce an adjusted dissolved oxygenconcentration from 0.001 to 0.02% by mass; and casting the molten steel.The phrase “adding at least La and/or Ce” described herein signifiesadding La, adding Ce, or adding both La and Ce. The phrase is used belowwith the same meaning. The fundamental idea of this process is allowingdissolved oxygen to remain to such a degree that a reaction of C withoxygen to form a CO gas does not take place during casting, andadjusting the surface energy between the molten steel and inclusions bythe action of the dissolved oxygen so as to inhibit aggregation ofinclusions and disperse fine La₂O₃ inclusions, Ce₂O₃ inclusions orLa₂O₃—Ce₂O₃ composite inclusions. When at least La and/or Ce is added toallow dissolved oxygen to remain, inclusions in an amount correspondingto the amount of the remaining dissolved oxygen are not formed.Furthermore, the present inventors have experimentally evaluated theaggregation behavior of inclusions in a molten steel by varying adissolved oxygen concentration after adding at least La and/or Ce to themolten steel. As a result, they have made the following discoveries:even when dissolved oxygen is substantially removed by deoxidation withat least La and/or Ce, La₂O₃ inclusions, Ce₂O₃ inclusions or La₂O₃—Ce₂O₃composite inclusions hardly aggregate in comparison with alumina typeinclusions; moreover, when the dissolved oxygen concentration is set at0.001% by mass or more, La₂O₃ inclusions, Ce₂O₃ inclusions orLa₂O₃—Ce₂O₃ composite inclusions are further refined with an increase inthe dissolved oxygen concentration. The above phenomena take place forthe following reasons. Both the effect of varying the composition fromalumina type inclusions to La₂O₃ inclusions, Ce₂O₃ inclusions orLa₂O₃—Ce₂O₃ composite inclusions and the effect of increasing adissolved oxygen concentration in the molten steel significantly lower asurface energy between the inclusions and the molten steel to inhibitaggregation of inclusions.

[0046] When a molten steel containing a large amount of dissolved oxygenis cast after decarburization without deoxidation, CO bubbles aregenerated during solidification to markedly lower the castability.Therefore, a deoxidizing agent such as Al has conventionally been addedto a molten steel subsequent to decarburization to deoxidize the moltensteel to such a degree that no dissolved oxygen substantially remains.However, because a steel sheet required to have workability has a lowcarbon concentration, a CO bubble formation reaction of the formula (2)

C+O=CO  (2)

[0047] hardly takes place during casting even when dissolved oxygenremains in the molten steel to some extent. The limit dissolved oxygenconcentration at which no CO bubbles are generated is about 0.006% bymass at a C concentration of 0.04% by mass and about 0.01% by mass at aC concentration of 0.01% by mass. Moreover, for an extra low carbonsteel having a still lower C concentration, no CO bubbles are generatedeven when dissolved oxygen is allowed to remain at a concentration ofabout 0.015% by mass. A continuous casting machine has recently beenequipped with an electromagnetic stirring apparatus within the mold, andCO bubbles are not trapped by the slab at a dissolved oxygenconcentration as high as, for example, 0.02% by mass when the moltensteel is stirred during solidification. Accordingly, a molten steel forsteel sheets having a C concentration of up to 0.01% by mass can be castwhile dissolved oxygen is allowed to remain at a concentration of about0.02% by mass. Conversely, when the dissolved oxygen concentrationexceeds 0.02% by mass, even a molten steel for steel sheets generates CObubbles.

[0048] Furthermore, when the dissolved oxygen concentration is lowered,the surface energy between a molten steel and inclusions cannot belowered greatly. As a result, even La₂O₃ inclusions, Ce₂O₃ inclusions orLa₂O₃—Ce₂O₃ composite inclusions gradually aggregate, and the inclusionsare partly coarsened. Experimental examination concludes that at least0.001% by mass of dissolved oxygen is required to prevent the inclusionsfrom coarsening.

[0049] Accordingly, the dissolved oxygen concentration of a moltensteel, which the carbon concentration of which has been set at 0.01% bymass or less, is restricted to from 0.001 to 0.02% by mass during addingat least Ce and/or La. That is, although addition of at least Ce and/orLa is effective in refining inclusions, addition of at least Ce and/orLa in a large amount markedly lowers the dissolved oxygen concentrationbecause the elements are very strong deoxidizing agents, and theinclusion refining effect of the invention is impaired. At least Laand/or Ce must therefore be added in such a range that the dissolvedoxygen remains at a concentration from 0.001 to 0.02% by mass.

[0050] Next, the present inventors have devised, as another aspect ofthe invention, a process comprising the steps of: decarburizing a moltensteel so as to produce a carbon concentration of up to 0.01% by mass, byrefining the molten steel with a steel making furnace such as aconverter or an electric furnace, or further subjecting the molten steelto vacuum degassing procedure or the like; adding Ti and at least Laand/or Ce thereto; and casting the molten steel.

[0051] The present inventors have used Al or Ti, and at least La and/orCe in suitable combinations as deoxidizing agents to be added to moltensteels, and experimentally evaluated the aggregation behavior of theseinclusions. As a result, they have made the following discovery: Al₂O₃inclusions, TiO_(n) inclusions, Al₂O₃—La₂O₃—Ce₂O₃ composite inclusions,Al₂O₃— La₂O₃ composite inclusions or Al₂O₃—Ce₂O₃ composite inclusionsrelatively easily aggregate; in contrast to the above inclusions,TiO_(n)—La₂O₃—Ce₂O₃ composite inclusions, TiO_(n)—La₂O₃ compositeinclusions or TiO_(n)—Ce₂O₃ composite inclusions hardly aggregate, andare finely dispersed in molten steels. The above phenomena take placefor the following reasons: the surface energy between a molten steel andany of the inclusions of TiO_(n)—La₂O₃—Ce₂O₃, TiO_(n)—La₂O₃ andTiO_(n)—Ce₂O₃ is greatly lower in comparison with the surface energybetween a molten steel and any of the inclusions of Al₂O₃, TiO_(n),Al₂O₃—La₂O₃—Ce₂O₃, Al₂O₃—La₂O₃ and Al₂O₃—Ce₂O₃, and aggregation of theinclusions is inhibited. On the basis of these discoveries, dissolvedoxygen in a molten steel is decreased by deoxidation with Ti, and atleast La and/or Ce is further added thereto to modify TiO_(n) inclusionsinto TiO_(n)—La₂O₃—Ce₂O₃ composite inclusions, TiO_(n)—La₂O₃ compositeinclusions or TiO_(n)—Ce₂O₃ composite inclusions.

[0052] As described above, inclusions in a molten steel can be finelydispersed by modifying oxide inclusions therein. The dissolved oxygenconcentration in a molten steel subsequent to addition of Ti, and atleast La and/or Ce therefore is not specifically defined. However, Ti,Ce and La are all deoxidizing agents, and addition of them to a moltensteel in a large amount greatly lowers a dissolved oxygen concentration.Accordingly, adding Ti, Ce and La so as to produce a dissolved oxygenconcentration in the range from 0.001 to 0.02% by mass is preferredbecause the effects of lowering the surface energy of the molten steeland avoiding aggregation of inclusions can be achieved.

[0053] Furthermore, the present inventors have devised, as anotheraspect of the invention, a process comprising the steps of:decarburizing a molten steel so as to produce a carbon concentration ofup to 0.01% by mass by refining the molten steel with a steel makingfurnace such as a converter or an electric furnace, or furthersubjecting the molten steel to vacuum degassing or the like procedure;pre-deoxidizing the molten steel by adding Al thereto so as to produce adissolved oxygen concentration from 0.01 to 0.04% by mass; adding Ti andat least La and/or Ce thereto; and casting the molten steel.

[0054] A more practical process is considered in view of the productioncost. Entire deoxidation with Al of a molten steel subsequent todecarburization is not conducted, but the molten steel is pre-deoxidizedwith Al so as to allow dissolved oxygen to remain; the resultant Al₂O₃inclusions are allowed to float and are removed in a short period oftime to such an extent that the inclusions do not exert adverse effects,and the molten steel is deoxidized again with an element other than Al.The process makes the improvement of the quality of a steel product andthe reduction of the production cost compatible.

[0055] As explained above, the present inventors have used Al or Ti, andat least La and/or Ce in suitable combinations as deoxidizing agents tobe added to molten steels, and experimentally evaluated the aggregationbehavior of these inclusions. They have elucidated the followingresults: Al₂O₃ inclusions, TiO_(n) inclusions, Al₂O₃—La₂O₃—Ce₂O₃composite inclusions, Al₂O₃—La₂O₃ composite inclusions or Al₂O₃—Ce₂O₃composite inclusions relatively easily aggregate; in contrast to theabove inclusions, TiO_(n)—La₂O₃—Ce₂O₃ composite inclusions,TiO_(n)—La₂O₃ composite inclusions or TiO_(n)—Ce₂03 composite inclusionshardly aggregate and are finely dispersed in molten steels. The presentinventors based on the above discoveries have been capable of formingTiO_(n)—La₂O₃—Ce₂O₃ composite inclusions, TiO_(n)—La₂O₃ compositeinclusions or TiO_(n)—Ce₂O₃ composite inclusions containing no Al₂O₃inclusions, and finely dispersing the inclusions in molten steels by thefollowing procedure: in place of deoxidizing a molten steel subsequentto decarburization with Ti alone, the molten steel is at firstpre-deoxidized with Al so that part of the dissolved oxygen is removed,and Al₂O₃ inclusions are allowed to float and are removed in a shortperiod of time by stirring and the like procedure to such an extent thatthe remaining Al₂O₃ inclusions exert no adverse effects; the moltensteel is deoxidized again with Ti so that the remaining dissolved oxygenis reduced; and at least La and/or Ce is further added. Surface defectson a steel sheet can thus be surely prevented by preventing formation ofaggregates of inclusions in the molten steel and finely dispersing theinclusions in the steel sheet. The concentration of the above remainingAl₂O₃ inclusion subsequent to Al pre-deoxidation that exert no adverseeffects is not specifically defined as long as surface defects on thesteel sheet is prevented. However, usually, the inclusion concentrationis as high as, for example, about 50 ppm or less.

[0056] Because La and Ce have very high deoxidation capabilities incomparison with Ti, TiO_(n) inclusions formed after adding Ti can beeasily modified into TiO_(n)—La₂O₃—Ce₂O₃ composite inclusions,TiO_(n)—La₂O₃ composite inclusions or TiO_(n)—Ce₂O₃ composite inclusionsby reducing the TiO_(n) inclusions with a small amount of Ce and/or La.However, when the dissolved oxygen concentration subsequent to Alpre-deoxidation exceeds 0.04% by mass, a large amount of TiO_(n)inclusions is formed after adding Ti. As a result, unmodified TiO_(n)inclusions partly remain even when La and/or Ce is added, and tend tobecome coarse titania clusters. On the other hand, when an additionamount of Al is increased to decrease a dissolved oxygen concentrationsubsequent to pre-deoxidation, a large amount of Al₂O₃ inclusions isformed. Accordingly, in order to decrease Al₂O₃ inclusions that arelikely to coarsen, as much as possible, it is preferred to set adissolved oxygen concentration subsequent to deoxidation with Al at0.01% by mass or more. Therefore, in the present invention, thedissolved oxygen concentration subsequent to pre-deoxidation with Al ispreferably adjusted to a range from 0.01% to 0.04% by mass.

[0057] Furthermore, Ti, Ce and La are all deoxidizing agents, andaddition of them to a molten steel in a large amount greatly decreases adissolved oxygen concentration. Adding Ti, Ce and La to produce adissolved oxygen concentration from 0.001 to 0.02% by mass is preferredbecause the effects of lowering the surface energy of the molten steeland avoiding aggregation of inclusions can be achieved.

[0058] Furthermore, it is desirable to allow Al not to remain in themolten steel for the purpose of forming no alumina type inclusions thatare likely to aggregate. However, Al may be allowed to remain when theamount is small. In this case, the dissolved oxygen must be allowed toremain in a molten steel in an amount of at least 0.001% by mass.According to thermodynamic calculation, the dissolved Al concentrationat 1,600° C. should be up to 0.005% by mass.

[0059] Still furthermore, the present inventors have devised, as adetailed aspect of the invention, a process comprising the steps of:decarburizing a molten steel so as to produce a carbon concentration ofup to 0.01% by mass by refining the molten steel with a steel makingfurnace such as a converter or an electric furnace, or furthersubjecting the molten steel to vacuum degassing procedure or the like;pre-deoxidizing the molten steel by adding Al thereto and stirring themolten steel for at least 3 minutes so as to produce a dissolved oxygenconcentration from 0.01 to 0.04% by mass; adding thereto Ti in an amountfrom 0.003 to 0.4% by mass and at least La and/or Ce in an amount from0.001 to 0.03% by mass; and casting the molten steel.

[0060] Experimental examination has clarified that most of Al₂O₃inclusions can be allowed to float and be removed by setting a dissolvedoxygen concentration subsequent to Al addition during thepre-deoxidation at 0.01% by mass or more, and ensuring a stirring timeof at least 3 minutes after Al addition. In particular, when a vacuumdegassing apparatus is employed, the molten steel is commonly circulatedas a stirring procedure after Al addition.

[0061] When the molten steel subsequent to pre-deoxidation is deoxidizedby adding a small amount of Ti, part of the dissolved oxygen remains inthe molten steel due to a weak deoxidation capability of Ti comparedwith Al. As explained above, when the dissolved oxygen concentrationexceeds 0.02% by mass in a molten steel for steel sheets having a Cconcentration of up to 0.01% by mass, CO bubbles are generated. Ti musttherefore be added to the molten steel so as to produce a dissolvedoxygen concentration of up to 0.02% by mass. The equilibrium calculationindicates that the Ti concentration is at least 0.003% by mass. On theother hand, although Ti is a deoxidizing agent having a relatively weakdeoxidation capability, addition of Ti to the molten steel in a largeamount greatly lowers a dissolved oxygen concentration of the moltensteel. As a result, even subsequent addition of at least La and/or Cehardly modifies the inclusions in the molten steel into compositeinclusions of TiO_(n)—La₂O₃—Ce₂O₃, TiO_(n)—La₂O₃ or TiO_(n)—Ce₂O₃.Therefore, the effect of refining inclusions of the invention isimpaired. The Ti concentration must therefore be set at 0.4% by mass orless to allow dissolved oxygen to remain at a concentration of aboutseveral ppm. It can be concluded from the above explanation that the Ticoncentration desirably is set at from 0.003% by mass or more to 0.4% bymass.

[0062] Addition of at least La and/or Ce is effective in refininginclusions. However, because La and Ce are strong deoxidizing agents,they react with refractories and mold flux to contaminate the moltensteel and deteriorate the refractories and mold flux. Therefore, theaddition amount of at least La and/or Ce is at least an amount necessaryfor modifying the TiO_(n) inclusions thus formed, and up to an amountnot to contaminate the molten steel by the reaction of La and Ce withrefractories and mold flux. It is concluded from the experimentalexamination that the proper range of the concentration of at least Laand/or Ce in the molten steel is from at least 0.001% by mass to 0.03%by mass. Moreover, La and/or Ce are not always added within a vacuumdegassing apparatus, and may be added to the molten steel after Ti isadded and before the molten steel is allowed to flow into a mold. Forexample, they may be added within a tundish. Moreover, La and/or Ce maybe added using pure La and/or Ce. They may also be added in the form ofan alloy containing La and/or Ce such as misch metal. Even when otherimpurities are mixed into the molten steel in combination with La and/orCe, the effect of the present invention is not impaired as long as atotal concentration of La and/or Ce in the alloy is at least 30% bymass.

[0063] Moreover, in the above process, the molten steel may bedecarburized with a vacuum degassing apparatus.

[0064] Furthermore, Ti, Ce and La are all deoxidizing agents, andaddition of them to a molten steel in a large amount greatly decreases adissolved oxygen concentration. Adding Ti, Ce and La to produce adissolved oxygen concentration from 0.001 to 0.02% by mass is preferredbecause the effects of lowering the surface energy of the molten steeland avoiding aggregation of inclusions can be achieved.

[0065] When a molten steel of the present invention is continuouslycast, La₂O₃ inclusions, Ce₂O₃ inclusions, La₂O₃—Ce₂O₃ compositeinclusions, TiO_(n)—La₂O₃ composite inclusions, TiO_(n)—Ce₂O₃ compositeinclusions and TiO_(n)—La₂O₃—Ce₂O₃ composite inclusions are absorbed inmold flux as the casting time passes, and there is a possibility of alowering of the mold flux viscosity. The lowering of the mold fluxviscosity promotes inclusion of the flux, and the inclusion causes ofmold flux-caused defects. When a molten steel of the invention is to becontinuously cast, it is therefore effective to design a higher moldflux viscosity in advance while viscosity lowering caused by absorptionof the inclusions is taken into consideration. Experimental results showthat defects caused by mold flux have not been formed when the viscosityof the mold flux at 1300° C. is set at 4 poise or more.

[0066] Furthermore, the mold flux has a function of lubricating amovement between the mold and the slab, and the upper limit of theviscosity is not particularly defined as long as the function is notimpaired.

[0067] The present invention can be applied to both ingot casting andcontinuous casting. For continuous casting, the present invention isapplied not only to continuous casting of an ordinary slab having athickness of about 250 mm but also to continuous casting of a thin slabwith a continuous casting machine having a smaller mold thickness of,for example, 150 mm or less to manifest sufficient effects and give aslab having extremely decreased surface defects.

[0068] Furthermore, steel sheets can be produced from the slabs obtainedby the above process by conventional procedures such as hot rolling andcold rolling.

[0069] Evaluation of the dispersed state of inclusions in the surfacelayer from the surface to the depth of 20 mm of a slab obtained by theprocess of the present invention has given the following results: fineoxide inclusions having a diameter from 0.5 to 30 μm are dispersedtherein with the number being from not less than 1,000 to 100,000pieces/cm². When inclusions are dispersed as fine oxide inclusions asexplained above, prevention of surface defects can be achieved. Thedispersed state of inclusions herein has been evaluated from aninclusion particle size distribution in a unit area byoptical-microscopically observing the ground surface of a slab or steelsheet at magnifications of 100 and 1,000. The particle size, namely, thediameter of an inclusion is obtained by measuring the major and minoraxes, and calculating from the formula

particle size (diameter)=(major axis×minor axis)^(0.5)

[0070] The major and minor axes used herein are the same as thoseusually used for an ellipse or the like.

[0071] Furthermore, when not less than 60% by mass of oxide inclusionspresent in the surface layer of the slab from the surface to the depthof 20 mm contain at least La and/or Ce, aggregation of inclusions isinhibited as explained above, and the effect of finely dispersinginclusions is achieved.

[0072] Moreover, the oxide inclusions are usually spherical orspindle-like oxide inclusions.

[0073] Furthermore, when not less than 60% by mass of oxide inclusionspresent in the surface layer of a slab from the surface to the depth of20 mm are oxide inclusions containing not less than 20% by mass,preferably not less than 40% by mass, more preferably not less than 55%by mass of at least La and/or Ce in the form of La₂O₃ and/or Ce₂O₃, theeffect of refining inclusions as explained above is produced.

[0074] Furthermore, the oxide inclusions are usually spherical orspindle-like oxide inclusions.

[0075] In addition, the present inventors have paid attention to thedistribution of inclusions in the surface layer from the surface to thedepth of 20 mm because it is highly possible that the inclusions in theabove range be exposed to the surface after rolling to form surfacedefects.

[0076] Furthermore, a steel sheet obtained by working a slab having theoxide inclusions that have such a dispersed state, a composition and ashape as explained above, for example, a hot rolled steel sheet obtainedby hot rolling, a cold rolled steel sheet obtained by further coldrolling the hot rolled steel sheet, and the like sheet are each definedas a steel sheet in the present invention.

[0077] It has been concluded from the evaluation of the dispersed stateof inclusions in a steel sheet that the dispersed state is substantiallythe same as that of oxide inclusions in the surface layer of a slab fromthe surface to the depth of 20 mm.

[0078] A steel sheet obtained by working a slab having such a dispersedstate, a composition and a shape of oxide inclusions as explained abovehas not formed surface defects. It is concluded from the above resultsthat because inclusions can be finely dispersed in a molten steel by thepresent invention, the inclusions cause no formation of surface defectsduring the production of a steel sheet, and the quality of a steel sheetis greatly improved.

EXAMPLES

[0079] The present invention will be explained by making reference toexamples and comparative examples.

Example 1

[0080] A molten steel in an amount of 300 tons in a ladle, having beenrefined in a converter and treated in a circulation type vacuumdegassing apparatus to have a carbon concentration of 0.003% by mass,was deoxidized with Ce to have a Ce concentration of 0.0002% by mass anda dissolved oxygen concentration of 0.0014% by mass. The molten steelwas continuously cast into slab steel having a thickness of 250 mm and awidth of 1,800 mm. The cast slab steel was cut to give slabs each havinga length of 8,500 mm (each slab being one coil unit). Each slab thusobtained was conventionally hot rolled and cold rolled to finally give acold rolled steel sheet in a coil having a thickness of 0.7 mm and awidth of 1,800 mm. The cold rolled steel sheet was visually observed onthe inspection line subsequent to cold rolling, and the slab quality wasevaluated from the number of surface defects formed per coil. As aresult, no surface defects were found.

Example 2

[0081] A molten steel in an amount of 300 tons in a ladle, having beenrefined in a converter and treated in a circulation type vacuumdegassing apparatus to have a carbon concentration of 0.003% by mass,was deoxidized with Ti and Ce to have a Ti concentration of 0.008% bymass, a Ce concentration of 0.0001% by mass and a dissolved oxygenconcentration of 0.0022% by mass. The molten steel was continuously castinto slab steel having a thickness of 250 mm and a width of 1,800 mm.The cast slab steel was cut to give slabs each having a length of 8,500mm (each slab being one coil unit). Each slab thus obtained wasconventionally hot rolled and cold rolled to finally give a cold rolledsteel sheet in a coil having a thickness of 0.7 mm and a width of 1,800mm. The cold rolled steel sheet was visually observed on the inspectionline subsequent to cold rolling, and the slab quality was evaluated fromthe number of surface defects formed per coil. As a result, no surfacedefects were found.

Example 3

[0082] Al for pre-deoxidation in an amount of 100 kg was added to 300tons of a molten steel in a ladle having been refined with a converterand treated with a vacuum degassing apparatus to have a carbonconcentration of 0.003% by mass, and the molten steel was circulated for3 minutes to have a dissolved oxygen concentration of 0.02% by mass. Tiin an amount of 200 kg was further added to the molten steel, and themolten steel was circulated for 1 minute. Thereafter, the additives Ce,La and 40 mass % La-60 mass % Ce each in an amount of 40 kg were addedto three separate molten steels each in a ladle, respectively. As aresult, one of the molten steels had a Ti concentration of 0.03% by massand a Ce concentration of 0.007% by mass. Another molten steel had a Ticoncentration of 0.03% by mass and a La concentration of 0.007% by mass.The other molten steel had a Ti concentration of 0.03% by mass and a Laconcentration and a Ce concentration in total of 0.007% by mass. Eachmolten steel was continuously cast into slab steel having a thickness of250 mm and a width of 1,800 mm. Mold flux used during casting had aviscosity of 6 poise. The cast slab steel was cut to give slabs eachhaving a length of 8,500 mm (each slab being one coil unit). Inclusionsin the surface layer from the surface to the depth of 20 mm of the slabwere examined. Each slab prepared by addition of Ce alone or La alone,or by composite addition of La—Ce had fine oxide inclusions from 0.5 to30 μm in diameter dispersed therein with the number being from 11,000 to13,000 pieces/cm². Seventy-five percent by mass of the fine oxideinclusions were spherical or spindle-like oxide inclusions containingnot less than 57% by mass of La₂O₃ alone, Ce₂O₃ alone, or La₂O₃ andCe₂O₃ in total. Each slab thus obtained was conventionally hot rolledand cold rolled to finally give a cold rolled steel sheet in a coilhaving a thickness of 0.7 mm and a width of 1,800 mm. The cold rolledsteel sheet was visually observed on the inspection line subsequent tocold rolling, and the steel sheet quality was evaluated from the numberof surface defects formed per coil. As a result, no surface defects wereformed in any of the coils each prepared by addition of Ce alone or Laalone, or by composite addition of La—Ce. Moreover, when inclusions inany of the cold rolled steel sheets each prepared by addition of Cealone or La alone, or by composite addition of La—Ce were examined, thesteel sheet had fine oxide inclusions from 0.5 to 30 μm in diameterdispersed therein with the number being from 11,000 to 13,000pieces/cm². Seventy-five percent by mass of the fine oxide inclusionswere spherical or spindle-like oxide inclusions containing not less than57% by mass of La₂O₃ alone, Ce₂O₃ alone, or La₂O₃ and Ce₂O₃ in total.

Example 4

[0083] Al for pre-deoxidation in an amount of 150 kg was added to 300tons of a molten steel in a ladle having been refined with a converterand treated with a vacuum degassing apparatus to have a carbonconcentration of 0.005% by mass, and the molten steel was circulated for5 minutes to have a dissolved oxygen concentration of 0.012% by mass. Tiin an amount of 250 kg was further added to the molten steel, and themolten steel was circulated for 2 minutes. Thereafter, the additives Ce,La and 40 mass % La-60 mass % Ce each in an amount of 100 kg were addedto three separate molten steels each in a ladle, respectively. As aresult, one of the molten steels had a Ti concentration of 0.045% bymass and a Ce concentration of 0.018% by mass. Another molten steel hada Ti concentration of 0.045% by mass and a La concentration of 0.018% bymass. The other molten steel had a Ti concentration of 0.045% by massand a La concentration and a Ce concentration in total of 0.018% bymass. Each molten steel was continuously cast into thin slab steelhaving a thickness of 70 mm and a width of 1,800 mm. Mold flux usedduring casting had a viscosity of 5 poise. The cast slab steel was cutto give slabs each having a length of 10,000 mm (each slab being onecoil unit). Inclusions in the surface layer from the surface to thedepth of 20 mm of the slab were examined. Each slab prepared by additionof Ce alone or La alone, or by composite addition of La—Ce had fineoxide inclusions from 0.5 to 30 μm in diameter dispersed therein withthe number being from 12,000 to 14,000 pieces/cm². Eighty percent bymass of the fine oxide inclusions were spherical or spindle-like oxideinclusions containing not less than 60% by mass of La₂O₃ alone, Ce₂O₃alone, or La₂O₃ and Ce₂O₃ in total. Each thin slab thus obtained wasconventionally hot rolled and cold rolled to finally give a cold rolledsteel sheet in a coil having a thickness of 0.7 mm and a width of 1,800mm. The cold rolled steel sheet was visually observed on the inspectionline subsequent to cold rolling, and the steel sheet quality wasevaluated from the number of surface defects occurred per coil. As aresult, no surface defects were occurred in any of the coils eachprepared by addition of Ce alone or La alone, or by composite additionof La—Ce. Moreover, when inclusions in any of the cold rolled steelsheets each prepared by addition of Ce alone or La alone, or bycomposite addition of La—Ce were examined, the steel sheet had fineoxide inclusions from 0.5 to 30 μm in diameter dispersed therein withthe number being from 12,000 to 14,000 pieces/cm². Eighty percent bymass of the fine oxide inclusions were spherical or spindle-like oxideinclusions containing not less than 60% by mass of La₂O₃ alone, Ce₂O₃alone, or La₂O₃ and Ce₂O₃ in total.

Example 5

[0084] Al for pre-deoxidation in an amount of 50 kg was added to 300tons of a molten steel in a ladle having been refined with a converterand treated with a vacuum degassing apparatus to have a carbonconcentration of 0.001% by mass, and the molten steel was circulated for3 minutes to have a dissolved oxygen concentration of 0.038% by mass. Tiin an amount of 80 kg was further added to the molten steel, and themolten steel was circulated for 2 minutes. Thereafter, the additives Ce,La and 30 mass % La-70 mass % Ce each in an amount of 30 kg were addedto three separate molten steels each in a ladle, respectively. As aresult, one of the molten steels had a Ti concentration of 0.01% by massand a Ce concentration of 0.005% by mass. Another molten steel had a Ticoncentration of 0.01% by mass and a La concentration of 0.005% by mass.The other molten steel had a Ti concentration of 0.01% by mass and a Laconcentration and a Ce concentration in total of 0.005% by mass. Eachmolten steel was continuously cast into slab steel having a thickness of250 mm and a width of 1,800 mm. Mold flux used during casting had aviscosity of 8 poise. The cast slab steel was cut to give slabs eachhaving a length of 8,500 mm (each slab being one coil unit). Inclusionsin the surface layer from the surface to the depth of 20 mm of the slabwere examined. Each slab prepared by addition of Ce alone or La alone,or by composite addition of La—Ce had fine oxide inclusions from 0.5 to30 μm in diameter dispersed therein with the number being from 8,000 to10,000 pieces/cm². Seventy-five percent by mass of the fine oxideinclusions were spherical or spindle-like oxide inclusions containingnot less than 58% by mass of La₂O₃ alone, Ce₂O₃ alone, or La₂O₃ andCe₂O₃ in total. Each slab thus obtained was conventionally hot rolledand cold rolled to finally give a cold rolled steel sheet in a coilhaving a thickness of 0.7 mm and a width of 1,800 mm. The cold rolledsteel sheet was visually observed on the inspection line subsequent tocold rolling, and the steel sheet quality was evaluated from the numberof surface defects occurred per coil. As a result, no surface defectswere occurred in any of the coils each prepared by addition of Ce aloneor La alone, or by composite addition of La—Ce. Moreover, wheninclusions in any of the cold rolled steel sheets each prepared byaddition of Ce alone or La alone, or by composite addition of La—Ce wereexamined, the steel sheet had fine oxide inclusions from 0.5 to 30 μm indiameter dispersed therein with the number being from 8,000 to 10,000pieces/cm². Seventy-five percent by mass of the fine oxide inclusionswere spherical or spindle-like oxide inclusions containing not less than58% by mass of La₂O₃ alone, Ce₂O₃ alone, or La₂O₃ and Ce₂O₃ in total.

Comparative Example 1

[0085] A molten steel in a ladle having been refined with a converterand treated with a circulation type vacuum degassing apparatus to have acarbon concentration of 0.003% by mass was deoxidized with Al to have anAl concentration of 0.04% by mass and a dissolved oxygen concentrationof 0.0002% by mass. The molten steel was continuously cast into slabsteel having a thickness of 250 mm and a width of 1,800 mm. The castslab steel was cut to give slabs each having a length of 8,500 mm (eachslab being one coil unit). Each slab thus obtained was conventionallyhot rolled and cold rolled to finally give a cold rolled steel sheet ina coil having a thickness of 0.7 mm and a width of 1,800 mm. The coldrolled steel sheet was visually observed on the inspection linesubsequent to cold rolling, and the slab quality was evaluated from thenumber of surface defects occurred per coil. As a result, the averagenumber of surface defects occurred per coil was 5 pieces/coil.

Comparative Example 2

[0086] A molten steel in a ladle having been refined with a converterand treated with a vacuum degassing apparatus to have a carbonconcentration of 0.003% by mass was deoxidized with Al to have an Alconcentration of 0.04% by mass and a dissolved oxygen concentration of0.0002% by mass. The molten steel was continuously cast into slab steelhaving a thickness of 250 mm and a width of 1,800 mm. The cast slabsteel was cut to give slabs each having a length of 8,500 mm (each slabbeing one coil unit). Inclusions in the surface layer from the surfaceto the depth of 20 mm of the slab were examined. As a result, fine oxideinclusions having a diameter from 0.5 to 30 μm were present in the slabwith the number being only 500 pieces/cm². Ninety-eight percent of theoxide inclusions were alumina clusters. Each slab thus obtained wasconventionally hot rolled and cold rolled to finally give a cold rolledsteel sheet in a coil having a thickness of 0.7 mm and a width of 1,800mm. The cold rolled steel sheet was visually observed on the inspectionline subsequent to cold rolling, and the steel sheet quality wasevaluated from the number of surface defects occurred per coil. As aresult, the average number of surface defects occurred per coil was 5pieces/coil. Moreover, when inclusions in the cold rolled steel sheetwere examined, fine oxide inclusions having a diameter from 0.5 to 30 μmwere present in the slab with the number being only 600 pieces/cm², and98% by mass of them were alumina clusters.

INDUSTRIAL APPLICABILITY

[0087] As explained above, because inclusions in a molten steel can bemade finely dispersed according to the present invention, a low carbonsteel sheet that can be surely prevented from occurring surface defectsand that is excellent in workability and formability can be produced.

1. (delete)
 2. A low carbon steel sheet characterized in that not lessthan 60% by mass of oxide inclusions present therein contain at least Laand/or Ce.
 3. A low carbon steel sheet characterized in that not lessthan 60% by mass of oxide inclusions present therein are spherical orspindle-like oxide inclusions containing at least La and/or Ce.
 4. A lowcarbon steel sheet characterized in that not less than 60% by mass ofoxide inclusions present therein are oxide ones containing not less than20% by mass of at least La and/or Ce in the form of La₂O₃ and/or Ce₂O₃.5. A low carbon steel sheet characterized in that not less than 60% bymass of oxide inclusions present therein are spherical or spindle-likeoxide inclusions containing not less than 20% by mass of at least Laand/or Ce in the form of La₂O₃ and/or Ce₂O₃.
 6. A low carbon steel sheetcharacterized in that fine oxide inclusions having a diameter from 0.5to 30 μm are dispersed therein with the number being from not less than1,000 to less than 100,000 pieces/cm², and that not less than 60% bymass of the oxide inclusions contain at least La and/or Ce.
 7. A lowcarbon steel sheet characterized in that fine oxide inclusions having adiameter from 0.5 to 30 μm are dispersed therein with the number beingfrom not less than 1,000 to less than 100,000 pieces/cm², and that notless than 60% by mass of the oxide inclusions are spherical orspindle-like oxide inclusions containing at least La and/or Ce.
 8. A lowcarbon steel sheet characterized in that fine oxide inclusions having adiameter from 0.5 to 30 μm are dispersed therein with the number beingfrom not less than 1,000 to less than 100,000 pieces/cm², and that notless than 60% by mass of the oxide inclusions are oxide inclusionscontaining not less than 20% by mass of at least La and/or Ce in theform of La₂O₃ and/or Ce₂O₃.
 9. A low carbon steel sheet characterized inthat fine oxide inclusions having a diameter from 0.5 to 30 μm aredispersed therein with the number being from not less than 1,000 to lessthan 100,000 pieces/cm², and that not less than 60% by mass of the oxideinclusions are spherical or spindle-like oxide inclusions containing notless than 20% by mass of La and/or Ce in the form of La₂O₃ and/or Ce₂O₃.10. (delete)
 11. A low carbon steel slab characterized in that not lessthan 60% by mass of oxide inclusions present in the surface layer of theslab from the surface to the depth of 20 mm contain at least La and/orCe.
 12. A low carbon steel slab characterized in that not less than 60%by mass of oxide inclusions present in the surface layer of the slabfrom the surface to the depth of 20 mm are spherical or spindle-likeoxide inclusions containing at least La and/or Ce.
 13. A low carbonsteel slab characterized in that not less than 60% by mass of oxideinclusions present in the surface layer of the slab from the surface tothe depth of 20 mm are oxide inclusions containing not less than 20% bymass of at least La and/or Ce in the form of La₂O₃ and/or Ce₂O₃.
 14. Alow carbon steel slab characterized in that not less than 60% by mass ofoxide inclusions present in the surface layer of the slab from thesurface to the depth of 20 mm are spherical or spindle-like oxideinclusions containing not less than 20% by mass of at least La and/or Cein the form of La₂O₃ and/or Ce₂O₃.
 15. A low carbon steel slabcharacterized in that fine oxide inclusions having a diameter from 0.5to 30 μm are dispersed in the surface layer of the slab from the surfaceto the depth of 20 mm with the number being from not less than 1,000 toless than 100,000 pieces/cm², and that not less than 60% by mass of theoxide inclusions contain at least La and/or Ce.
 16. A low carbon steelslab characterized in that fine oxide inclusions having a diameter from0.5 to 30 μm are dispersed in the surface layer of the slab from thesurface to the depth of 20 mm with the number being from not less than1,000 to less than 100,000 pieces/cm², and that not less than 60% bymass of the oxide inclusions are spherical or spindle-like oxideinclusions containing at least La and/or Ce.
 17. A low carbon steel slabcharacterized in that fine oxide inclusions having a diameter from 0.5to 30 μm are dispersed in the surface layer of the slab from the surfaceto the depth of 20 mm with the number being from not less than 1,000 toless than 100,000 pieces/cm², and that not less than 60% by mass of theoxide inclusions are oxide inclusions containing not less than 20% bymass of at least La and/or Ce in the form of La₂O₃ and/or Ce₂O₃.
 18. Alow carbon steel slab characterized in that fine oxide inclusions havinga diameter from 0.5 to 30 μm are dispersed in the surface layer of theslab from the surface to the depth of 20 mm with the number being fromnot less than 1,000 to less than 100,000 pieces/cm², and that not lessthan 60% by mass of the oxide inclusions are spherical or spindle-likeoxide inclusions containing not less than 20% by mass of at least Laand/or Ce in the form of La₂O₃ and/or Ce₂O₃.
 19. A process for producinga low carbon steel slab comprising the steps of: decarburizing a moltensteel so as to produce a carbon concentration of up to 0.01% by mass;adding at least La and/or Ce thereto so as to produce an adjusteddissolved oxygen concentration from 0.001 to 0.02% by mass; and castingthe molten steel.
 20. A process for producing a low carbon steel slabcomprising the steps of: decarburizing a molten steel so as to produce acarbon concentration of up to 0.01% by mass; adding thereto Ti and atleast La and/or Ce; and casting the molten steel.
 21. A process forproducing a low carbon steel slab comprising the steps of: decarburizinga molten steel so as to produce a carbon concentration of up to 0.01% bymass; pre-deoxidizing the molten steel by adding Al thereto so as toproduce a dissolved oxygen concentration from 0.01 to 0.04% by mass;adding thereto Ti and at least La and/or Ce; and casting the moltensteel.
 22. A process for producing a low carbon steel slab comprisingthe steps of: decarburizing a molten steel so as to produce a carbonconcentration of up to 0.01% by mass; pre-deoxidizing the molten steelby adding Al thereto and stirring the molten steel for at least 3minutes so as to produce a dissolved oxygen concentration from 0.01 to0.04% by mass; adding thereto Ti in an amount from 0.003 to 0.4% by massand at least La and/or Ce in an amount from 0.001 to 0.03% by mass; andcasting the molten steel.
 23. A process for producing a low carbon steelslab comprising the steps of: decarburizing a molten steel with a vacuumdegassing apparatus so as to produce a carbon concentration of up to0.01% by mass; adding at least La and/or Ce thereto so as to produce anadjusted dissolved oxygen concentration from 0.001 to 0.02% by mass; andcasting the molten steel.
 24. A process for producing a low carbon steelslab comprising the steps of: decarburizing a molten steel with a vacuumdegassing apparatus so as to produce a carbon concentration of up to0.01% by mass; adding thereto Ti and at least La and/or Ce; and castingthe molten steel.
 25. A process for producing a low carbon steel slabcomprising the steps of: decarburizing a molten steel with a vacuumdegassing apparatus so as to produce a carbon concentration of up to0.01% by mass; pre-deoxidizing the molten steel by adding Al thereto soas to produce to a dissolved oxygen concentration from 0.01 to 0.04% bymass; adding thereto Ti and at least La and/or Ce; and casting themolten steel.
 26. A process for producing a low carbon steel slabcomprising the steps of: decarburizing a molten steel with a vacuumdegassing apparatus so as to produce a carbon concentration of up to0.01% by mass; pre-deoxidizing the molten steel by adding Al thereto andstirring the molten steel for at least 3 minutes so as to produce adissolved oxygen concentration from 0.01 to 0.04% by mass; addingthereto Ti in an amount from 0.003 to 0.4% by mass and at least Laand/or Ce in an amount from 0.001 to 0.03% by mass; and casting themolten steel.
 27. The process for producing a low carbon steel slabaccording to any one of claims 19 to 26 wherein, during casting themolten steel, the molten steel is cast in a mold having anelectromagnetic stirring function.
 28. The process for producing a lowcarbon steel slab according to any one of claims 19 to 26 wherein,during casting the molten steel, the molten steel is cast using moldflux having a viscosity of not lower than 4 poise at 1,300° C.
 29. Theprocess for producing a low carbon steel slab according to any one ofclaims 19 to 26 wherein, during casting the molten steel, the moltensteel is cast in a mold having an electromagnetic stirring functionusing mold flux having a viscosity of not lower than 4 poise at 1,300°C. 30 The process for producing a low carbon steel slab according to anyone of claims 19 to 26 wherein, during casting the molten steel, themolten steel is continuously cast.
 31. The process for producing a lowcarbon steel slab according to any one of claims 19 to 26 wherein,during casting a molten steel, the molten steel is continuously cast ina mold having an electromagnetic stirring function.
 32. The process forproducing a low carbon steel slab according to any one of claims 19 to26 wherein, during casting the molten steel, the molten steel iscontinuously cast using mold flux having a viscosity of not lower than 4poise at 1,300° C.
 33. The process for producing a low carbon steel slabaccording to any one of claims 19 to 26 wherein, during casting themolten steel, the molten steel is continuously cast in a mold having anelectromagnetic stirring function using mold flux having a viscosity ofnot lower than 4 poise at 1,300° C.